Selective extraction of nickel(II) and copper(II) by means of imidazole- and pyrazole-based pyridine ligands

Pearce, Brendan Harold (2017-03)

Thesis (MSc)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: In this study, imidazolyl- and pyrazolylpyridine ligands, along with sodium dodecylbenzenesulfonate (SDBS) as synergist, was investigated as potential selective extractants of nickel(II) and copper(II) from base metal ions in a solvent extraction system. The synthesis of the imidazolyl ligands, 2-(1H-imidazol-2-yl)pyridine (1), 2-(1-methyl-1H-imidazol-2- yl)pyridine (2), 2-(1-butyl-1H-imidazol-2-yl)pyridine (3) and 2-(1-octyl-1H-imidazol-2-yl)pyridine (4), followed the classic Debus-Radziszewski synthetic approach for imidazoles. The methylpyrazolyl ligands, 2- [(1H-pyrazol-1-yl)methyl]pyridine (5), 2-[(3,5-dimethyl-1H-pyrazol-1-yl)methyl]pyridine (6) and 2-[(3- methyl-1H-pyrazol-1-yl]methyl)pyridine / 2-[(5-methyl-1H-pyrazol-1-yl)methyl]pyridine (7/7’), were synthesised via simple nucleophilic substitution reactions (SN2 mechanism), while the pyrazolyl ligands, 2- (3-butyl-1H-pyrazol-5-yl)pyridine (8), 2-[3-(tert-butyl)-1H-pyrazol-5-yl]pyridine (9) and 2-(3-octyl-1Hpyrazol- 5-yl)pyridine (10) were obtained by the Claisen condensation of ethyl 2-picolinate with the appropriate alkyl ketones, followed by the classic Knorr synthesis for pyrazoles. Ligands 1–7/7’ were obtained in yields which ranged from 42.0–89.7%. Ligands 8–10 were obtained in particularly low yields (29.6, 26.1 and 26.3% respectively) due to the formation of unwanted side-products and the rigorous subsequent purification procedures. All ligands were characterised by 1H and 13C NMR, IR, mass spectrometry and elemental analysis. The extraction of nickel(II) and copper(II) from borderline hard/soft metal ions; Cd2+, Co2+, Pb2+ and Zn2+ was carried out at pH ≈ 5, using minute quantities of concentrated nitric acid and sodium hydroxide to adjust the pH when necessary. The optimum synergist concentration was found to be 0.05 M (5 times that of ligand and individual metal ion) after a range of optimisation studies were conducted. Nickel(II) extraction yielded results in the low to mid-70% range when ligands 1–10 were used in conjunction with SDBS, while extremely poor results were obtained in the absence of SDBS (most were < 10%). Time-dependent studies were conducted to prove that extraction equilibrium was reached well before the 24-hour mark. This was followed by a comprehensive competitive extraction study using ligands 1–10, whereby imidazolyl ligands, 1–3, displayed significant synergistic gains for copper(II) extraction, with 32.2 (± 1.0), 35.1 (± 0.9) and 54.1 (± 0.9)% respectively. Methylpyrazolyl ligands, 5–7/7’, also yielded good synergistic gains of 34.1 (± 0.5), 43.2 (± 0.3) and 39.0 (± 0.8)% respectively, while pyrazolyl ligands, 8–10, on the other hand, had negative synergistic interactions, with copper(II) being extracted in the mid-40% range. Ligands 8 and 10, however, exhibited impeccable copper(II) extractions in a mixed base metal ion environment, without the use of SDBS. Ligands 8 and 10 extracted 83.2 (± 0.6) and 90.2 (± 0.1)% copper(II) respectively, with selectivity studies corroborating these findings. Metal stripping studies of ligands 8–10, were somewhat underwhelming, with 46.7 (± 2.7), 55.0 (± 3.4) and 14.1 (± 1.8)% copper(II) being stripped at pH 1, respectively. Ligands 8–10 were also used for nickel(II) and copper(II) pH isotherm studies, whereby the optimum extraction and stripping ranges were established. Across the board, both nickel(II) and copper(II) were optimally extracted in the 4–6 pH range, while stripping occurred at pH < 3. Finally, we managed to grow single crystals of the aqua-2-[3-(tert-butyl)-1H-pyrazol-5-yl]-pyridine di(nitrato) copper(II) complex and solved the structure using single crystal X-ray diffraction. Coordination around the copper(II) centre is pseudo square pyramidal and indications are that the extraction stoichiometry is 1:1.

AFRIKAANSE OPSOMMING: Hierdie studie stel ondersoek in aangaande die selektiewe ekstraksie van nikkel(II) en koper(II) deur middel van imidasool- en pirasoolpiridienligande in samewerking met natriumdodesielbenseensulfonaat (NDBS). Die sintese van die imidasoolligande, 2-(1H-imidasol-2-iel)piridien (1), 2-(1-metiel-1H-imidasol-2- iel)piridien (2), 2-(1-butiel-1H-imidasol-2-iel)piridien (3) en 2-(1-oktiel-1H-imidasol-2-iel)piridien (4), was volgens die klassieke Debus-Radziszewski metode uitgevoer met goeie welslae. Die metielpirasoolligande, 2-[(1H-pirasol-1-iel)metiel]piridien (5), 2-[(3,5-dimetiel-1H-pirasol-1-iel)metiel]piridien (6) en 2-[(3-metiel- 1H-pirasol-1-iel]metiel)piridien / 2-[(5-metiel-1H-pirasol-1-iel)metiel]piridien (7/7’), is gesintetiseer deur middel van eenvoudige nukleofiliese substitusiereaksies (SN2 meganisme), terwyl die pirasoolligande, 2-(3- butiel-1H-pirasol-5-iel)piridien (8), 2-[3-(ters-butiel)-1H-pirasol-5-iel]piridien (9) en 2-(3-oktiel-1H-pirasol- 5-iel)piridien (10), vekry is deur die Claisen-kondensasie van etielpiridien-2-karboksilaat met die gepaste alkielketone, gevolg deur die klassieke Knorr-pirasoolsintese. Die opbrengste van ligande 1–7/7’ het gewissel tussen 42.0 en 89.7%, terwyl die ooglopende lae opbrengste van ligande 8–10 (29.6, 26.1 en 26.3% onderskeidelik) hoofsaaklik toegeskryf kon word aan die oplewering van ongewenste neweprodukte en die gepaardgaande suiweringsprosesse. Alle ligande is suksesvol gekarakteriseer deur 1H en 13C KMR, IR, massa spektrometrie en elementele analise. Die ekstraksie van nikkel(II) en koper(II) vanuit ‘n mengsel semi-hard en -sag metaalione (Cd2+, Co2+, Pb2+ en Zn2+) is by pH 5 uitgevoer, waartydens gekonsentreerde salpetersuur en natriumhidroksied met tye in klein maat gebruik is om die pH te reguleer. Die optimale sinergiskonsentrasie van 0.05 M (5 keer meer as die ligand en individuele metaalione) is aanvanklik bepaal en deurgaans as sulks gebruik. Nikkel(II)- ekstraksiestudies het goeie ekstraksieresultate opgelewer (70–78%) wanneer ligande 1–10 in die teenwoordigheid van NDBS gebruik is, terwyl beroerde resultate in die afwesigheid van NDBS verkry is (< 10%). Derhalwelik, is tydafhanklike studies ook uitgevoer om te verseker dat ekstraksie-ekwilibrium bereik is binne die 24-uurmerk. Dit was opgevolg deur ‘n omvattende mededingende-ekstraksiestudies deur gebruik te maak van ligande 1–10, waarbenewens imidasoolligande, 1–3, duidelike sinergiswinste vir koper(II)- ekstraksies getoon het (32.2 ± 1.0, 35.1 ± 0.9 en 54.1 ± 0.9% onderskeidelik). Die metielpirasoolligande, 5– 7/7’, het op sigself ook goeie sinergiswinste van 34.1 (± 0.5), 43.2 (± 0.3) en 39.0 (± 0.8)% getoon, terwyl pirasoolligande, 8–10, negatiewe sinergistiese interaksies tydens koper(II)-ekstraksies opgelewer het (40– 50%). Aan die ander kant, het ligande 8 en 10 uitstekende koper(II)-ekstraksieresultate opgelewer in die afwesigheid van NDBS. Hierdie twee ligande het koper(II) teen 83.2 (± 0.6) en 90.2 (± 0.1)% geëkstraheer, waarbenewens die selektiwiteitsstudies ook hierdie bevindinge gestaaf het. Ligande 8–10 is ondermeer ook gebruik vir metaalstropingsstudies by pH 1, maar ietwat teleurstellende resultate is verkry vanaf die koper(II)-stropingstudies (46.7 ± 2.7, 55.0 ± 3.4 en 14.1 ± 1.8%). Verder, was ligande 8–10 ook vir pHisotermstudies gebruik, waartydens die optimale pH-gebiede vir ekstraksies en stropings bepaal is. Daar was bevind dat bykans alle nikkel(II)- en koper(II)-ekstraksies optimaal gefunksioneer het by pH 4–6, terwyl stroping optimaal by pH < 3 gefunksioneer het. Die kristal- en molekulêre struktuur van akwa-2-[3-(ters-butiel)-1H-pirasol-5-iel]piridien di(nitrato) koper(II) is verkry deur middel van enkelkristal-X-straaldiffraksie-ontleding. Hiermee het ons bepaal dat ‘n pseudo vierkantig-piramidale kompleks gevorm is, met sekere trigonaal-bipiramidale eienskappe. Die mees belangrikste brokkie inligting wat hieruit verkry is, was die feit dat ligand 9 en die koper(II)-ioon in ‘n 1:1 stoigiometriese verhouding verkeer het.

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